1. Field of the Invention
The present invention relates to a stiffness detector that detects stiffness of a paper sheet such as a paper currency, a stiffness detection method, and a paper sheet processor including the stiffness detector.
2. Description of the Related Art
A paper sheet processor that counts and discriminates various kinds of paper sheets such as a paper currency have been put to practical use. The paper sheet processor takes paper sheets put in an injection module one by one and carries them to an examination device for paper sheets. The examination device executes various kinds of processing with respect to paper sheets to judge states of the paper sheets. The paper sheet processor executes a judgment on a type of each paper sheet, a true-false judgment, a wear judgment of judging whether a paper sheet can be again circulated, and others based on an examination result obtained by the examination device.
The examination device detects mechanical characteristics such as a degree of degradation in stiffness of paper sheets. The paper sheet processor determines paper sheets having degraded stiffness as paper sheets that are not suitable for recirculation.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2006-250869 as a Japanese patent document discloses a technology for applying ultrasonic waves to a paper sheet and detecting a characteristic frequency of the paper sheet from radiation waves from the paper sheet to judge degradation in stiffness of the paper sheet.
Further, Jpn. Pat. Appln. KOKAI Publication No. 1996-273020 as a Japanese patent document discloses a technology for dynamically measuring both or one of a thickness and a weighing capacity of a moving material. Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 2001-330423 as a Japanese patent document discloses a technology for detecting a state that two paper sheets have been taken at the same time based on a phase shift and an amplitude of ultrasonic waves. Moreover, Japanese patent No. 3993366 as a Japanese patent document discloses a technology for installing a wave transmitter and a wave receiver to sandwich a carrier path and detecting a state that two paper sheets have been taken at the same time based on acoustic waves that enter the wave receiver.
Additionally, Jpn. Pat. Appln. KOKAI Publication No. 2007-10638 and Jpn. Pat. Appln. KOKAI Publication No. 2008-164394 as a Japanese patent document discloses a technology for causing ultrasonic waves to enter a sample such as a metal plate, receiving leaky waves of waves that propagate through the sample and detecting a defect in the sample based on an amplitude of a received waveform.
The paper sheet processor carries paper sheets at a high speed and performs examination during carriage. However, the technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2006-250869 has a problem that measurement and an arithmetic operation require time since a characteristic frequency of a general paper sheet is low.
Further, according to the technologies disclosed in Jpn. Pat. Appln. KOKAI Publication No. 1996-273020, Jpn. Pat. Appln. KOKAI Publication No. 2001-330423 and Japanese patent No. 3993366, a paper sheet is irradiated with ultrasonic waves and, e.g., a thickness, a weight, multiplicity, or presence/absence of foreign particles of the paper sheet is detected based on reflected or transmitted waves. These technologies do not provide a configuration for measuring an elastic modulus (stiffness) of the paper sheet. Therefore, they have a problem that a degree of degradation in stiffness of the paper sheet cannot be judged.
Furthermore, the technologies disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2007-10638 and Jpn. Pat. Appln. KOKAI Publication No. 2008-164394 have a problem that a factor of a defect cannot be specified when detecting the defect in a sample. That is, they have a problem that a factor of a detected defect which may be a reduction in an elastic modulus, breakage, crack, or crease cannot be determined.
Additional objects and advantages of the embodiments will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.